This invention relates generally to measurement of angles turned in a threaded fastener tightening sequence and more particularly to a method of determining the angle with a non-contact reference. U.S. Pat. No. 4,211,120 issued to Angelo L. Tambini and assigned to SPS Technologies, Inc., and now Ingersoll-Rand Company, describes a tightening apparatus of a nature wherein the present invention finds application.
In the above identified prior art patent, which is incorporated herein by way of reference, a potentiometer 24 operates to provide an electrical output signal which is proportional to the rotational displacement of the driver head. A cable 26 and clip 28 is attached to a work piece to provide a fixed reference point.
In another embodiment, illustrated in FIG. 5, a digital angle measuring means 29 is disclosed. See column 3, lines 49 et. seq. The digital measuring means includes a high inertial disk as a reference. The fixed reference arm system is difficult to use, requires external fittings and increases both the size and complexity of the wrench head. Drift in the inertial disk non-contact reference created the possibility of accuracy problems and also added to the size, complexity, and cost of the wrench head. Recently angular velocity sensors, such as those supplied by the Ono-Sendai Corporation of San Francisco, Claif. and Murata Erie North American Inc. have become available. Mounting one of these on the wrench and integrating the signal will give a signal proportional to the angle turned by the wrench. The sensors, however, have problems with zero drift which cause inaccuracies in the term of a typical fastener tightening cycle.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.